1. Field of the Invention
The present invention relates to devices for reception of analog signals of the telephonic type, and in particular an opto-coupling device between an input circuit for analog signals of the telephonic type and an apparatus for signal processing such as a modem.
2. Description of the Prior Art
Information is more and more often transmitted over public network telephone lines, on which digital data are sent after having been converted to analog form thru an apparatus for signal processing such as a modem. On the receive side, analog signals incoming from the telephone line are converted back into digital data thru another modem.
Modems are relatively complex systems which require their own power supply coming from the AC central power. This can result in a dangerous situation in case of bad electrical contact on the telephone line which is powered from the central telephone branch exchange with a DC voltage (48 V in general) modulated by the transmitted analog signals. This is the reason why current regulations impose a coupling with no metallic contact between the telephone line and the apparatus for signal processing such as a modem, which is supplied by AC central power.
The known way of complying with the regulations is then to implement a transformer as interface between the telephone line and the modem. But it is very difficult to come up with transformers of a miniaturised size in proportion with the rest of electronic circuits which have also more and more reduced in size thanks to technological advancement in the last ten years, the ultimate goal being to integrate all line interface circuitry on a card the size of which complies with the PCMCIA (Personal Computer Memory Card International Association) standard, which is the size of a memory card, or a credit card.
One has then envisaged to utilise an opto-coupling system where electric current variations of the analog signal are applied to a light emitting diode (LED) which transforms them into light emission variations. The latter light emission then activates a phototransistor the output current of which varies according to the received light flow and therefore according to the variations of the analog signal.
Unfortunately, it proves impossible to use a diode of the LED type directly connected to the input circuitry for the analog signal. Indeed, the diode has a low impedance and a direct connection to the input circuitry would result in a drop of the input impedance of the telephone line which, for line adaptation reasons, needs stay at a constant value of generally 600 ohms, or any other value according to national regulations.